1. Field of the Invention
The present invention relates to a process for highly efficient production of carbon nanotubes with limited bundle growth and a highly precise arrangement, to high-quality carbon nanotubes, produced by said process, which are each individually and independently arranged in a highly precise manner at prescribed locations, and to a carbon nanotube production catalyst which is suitable for production of said carbon nanotubes.
The present invention also relates to a process for the production of carbon nanotubes on a substrate which is inclined from a highly symmetrical crystal orientation, or an “off-substrate”.
2. Description of the Related Art
Carbon nanotubes exhibit a variety of superior properties such as chemical stability, metallic and semiconductor electric conductivity, high electron emission, high mechanical strength (high elastic modulus) and high thermal conductivity. It is expected that utilization of such properties will make possible applications in a wide range of fields including electric field emission electron elements, scanning probe microscope (SPM) probes, catalysts, structural reinforcing materials, battery electrodes, sensor materials and the like. Considerable research is therefore being conducted toward chiral control and growth location control of carbon nanotubes.
Carbon nanotubes represent a relatively new form of carbon material, and they have recently been the object of a great deal of interest because of their unique properties. Carbon nanotubes have a structure wherein graphene sheets consisting of 6-membered carbon rings, bonded by the strongest type of bond (sp2), are rolled into a cylinder with the ends of the tubes closed by 5-membered rings interspersed between the 6-membered rings. The tube diameter can be reduced to the subnanometer order, with a minimum of 0.4 nanometer. Carbon nanotubes are also known to have various additional properties such as high current density (109 amperes per square centimeter) and ballistic conductivity, as well as high thermal conductivity and high mechanical strength, all combined with the advantage of their nanometer-scale diameter. FIGS. 14(a)-(c) are schematic representations of the different chiralities of carbon nanotubes. Carbon nanotubes exhibit semiconductor or metallic conductivity depending on the helical structure (the twist, or “chirality”) of the graphite. FIG. 14(a) is known as the “armchair type”, which exhibits metallic conductivity, FIG. 14(b) is known as the “zigzag type”, which exhibits semiconductor conductivity, and FIG. 14(c) is known as the “chiral type”, which exhibits conductivity between metallic and semiconductor. Application of such materials to electronic devices requires techniques for growing nanotubes with prescribed properties (chirality), having the prescribed orientation and at prescribed locations.
As processes for growing carbon nanotubes there may be mentioned arc discharge, laser vaporization, thermal CVD and plasma CVD. These processes can produce Single Wall Carbon Nanotubes (SWNTs) composed of a single graphene sheet and Multi Wall Carbon Nanotubes (MWNTs) composed of multiple graphene sheets. All of the processes require a metal catalyst (Fe, Co, Ni) for growth of the carbon nanotubes.
Growth of carbon nanotubes having a consistent orientation at prescribed locations is being investigated. Control of the location of growth of carbon nanotubes is accomplished primarily by arranging the metal catalyst at the prescribed location. In thermal CVD or plasma CVD, for example, the metal catalyst is pre-patterned onto a substrate, and an oriented electric field is applied to grow the carbon nanotubes in a consistent orientation at the prescribed locations.
While patterning of the metal catalyst in this manner can grow carbon nanotubes with a consistent orientation at the prescribed locations, the currently employed patterning methods only allow the metal catalyst patterns to be separated at sizes on the order of a few micrometers to a few hundred nanometers. Consequently, as shown in FIG. 15, numerous carbon nanotubes 3 with a diameter of from a few to a few hundred nanometers grow in a disordered fashion on each pattern of the metal catalyst 2 on the substrate 1, and often the carbon nanotubes grow in bundles with the molecules bonded together by van der Waals forces. It has been difficult by existing techniques to separate each of the individual carbon nanotubes which have grown in such bundles, and therefore the inability to use each of the individual carbon tubes has been a drawback.
The fluctuation in patterns resulting from lithography has constituted a problem in that it results in fluctuations in the border regions of nanotube growth. No success has yet been achieved in controlling chirality, and the development of a suitable method is desired.
The present invention overcomes these problems of the prior art and achieves the objects stated below. Specifically, it is an object of the invention to provide a process for production of carbon nanotubes with limited bundle growth and being independently arranged in a highly precise manner at prescribed locations, as well as high-quality carbon nanotubes produced by said process which are each individually and independently arranged in a highly precise manner at prescribed locations, and a carbon nanotube production catalyst which is suitable for production of said carbon nanotubes.
As explained above, no production process has yet been developed for carbon nanotubes having controlled growth location, diameter, orientation and chirality. It is therefore another object of the present invention to provide a production process for carbon nanotubes having controlled growth location, diameter, orientation and chirality.